Elasticsearch nodes discover their peers via multicast when they are in the same network. This works very well in a regular LAN. When it comes to EC2, the network is shared and the node to node communication and automatic discovery don't work. It requires the installation of the EC2 discovery plugin on all nodes to be able to discover new nodes.

To install the EC2 discovery plugin, follow the instructions at https://www.elastic.co/guide/en/elasticsearch/plugins/current/discovery-ec2.html and install it on all nodes.